ALUNITE (from the French word "alun", Latin word "alumen" which means "alums" * EN: alunite; DE: Alunit, Alaunstein; FR: alunite; ES: alunita; RU: alunite), the alum stone, is the mineral of the sulfates class, KaAl3[SO4]2(OH)6. It contains 10.0-10.5% of K2O, up to 40% of Al2O3, and not less than 35.5% of SO3. The admixtures are: Na (2.8-4.4% of Na2O in the natroalunite), sometimes Fe3+, Zn, and others.
Alunite crystallizes in the trigonal crystal system. The crystal structure is represented by the layers of the AlO2(OH)4 octahedra, which are interconnected by the K+ ions. Alunite crystals are rare, dense and loose masses are more common, fibrous and granular aggregates are less common. The colour is white, depending on the solid admixtures, grey, yellow, brown, green. The lustre is glassy, to the pearly. The hardness is 3.5-4.5; it is brittle. The density is 2700±100 kilograms per cubic metre. It is non-hygroscopic. The pyroelectric properties are strongly manifested. It appears as the result of the changes of the siliceous rocks by the sulfatic solutions: as the product of the solfatara activity in the volcanic regions (the main industrial type of the deposits), in the near-veinous zones of the hydrothermal deposits, and as the supergene mineral (in the weathering crusts and oxidation zones of the ore deposits).
Alunite is the raw material for the obtainment of the alums, aluminium sulfate, partially potassium salts and sulfuric acid. It is the potential source of aluminium. It is beneficiated by the flotation. The collectors are the saturated carboxylic acids; the regulators of the environment are the soda, NaOH, liquid glass; the activators are the lead ions; the depressors are the excess of the liquid glass, OH-ions at the pH>10. See also the "Alunite ore" article.
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